1. Field of the Invention
The present invention relates to high-speed data communication cables. More particularly, it relates to a high-speed data communication cable that uses a mechanical fixture to stabilize and control the physical layout of twisted pairs in the detwisted segment of the cable jacket so as to reduce crosstalk between the wire pairs.
2. Related Art
High speed data communications cables in current usage include pairs of wire twisted together forming a balanced transmission line. Such pairs of wire are referred to as twisted pairs.
One common type of conventional cable for high-speed data communications includes multiple twisted pairs within it. In each twisted pair, the wires are twisted together in a helical fashion, thus forming a balanced transmission line. Twisted pairs that are placed in close proximity, such as within a cable, may transfer electrical energy from one pair of the cable to another. Such energy transfer between pairs is undesirable and is referred to as crosstalk. Crosstalk is electromagnetic noise coupled to a twisted pair from an adjacent twisted pair, or from an adjacent cable. Telecommunications systems contain noise that interferes with the transmission of information. Crosstalk increases the interference to the information being transmitted through the twisted pair. The increased interference due to crosstalk can cause an increase in the occurrence of data transmission errors and a concomitant decrease in the data transmission rate. The Telecommunications Industry Association (TIA) and Electronics Industry Association (EIA) have defined standards for crosstalk in a data communications cable that include: TIA/EIA 568-A-2, published Aug. 14, 1998. The International Electrotechnical Commission (IEC) has also defined standards for data communications cable crosstalk, including ISO/IEC 11801 that is the international equivalent to TIA/EIA 568-A. One high performance standard for data communications cable is ISO/IEC 11801, Category 5.
Crosstalk is primarily capacitively coupled or inductively coupled energy passing between adjacent twisted pairs within a cable. Among the factors that determine the amount of crosstalk energy coupled between the wires in adjacent twisted pairs, the center-to-center distance between the wires in the adjacent twisted pairs is very important. The center-to-center distance is defined herein to be the distance between the center of one wire of a twisted pair to the center of another wire in an adjacent twisted pair. The magnitude of both capacitively coupled and inductively coupled crosstalk is inversely proportional to the center-to-center distance between wires. Increasing the distance between twisted pairs can thus reduce the level of crosstalk interference. Another factor relating to the level of crosstalk is the distance over which the wires run parallel to one another. Twisted pairs that have longer parallel runs typically have higher levels of crosstalk occurring between them.
In twisted pairs, the rate of the twist is known as the twist lay, and it is the distance between adjacent twists of the wire. The direction of the twist of a twisted pair is known as the twist direction. Adjacent twisted pairs having the same twist lay and/or opposing twist directions tend to lie more closely together within a cable than if they have different twist lays and/or same twist directions. Thus, compared to twisted pairs having different twist lays and/or same twist directions, adjacent twisted pairs having the same twist lay and opposing directions have a reduced center-to-center distance, and longer parallel run. Therefore, the level of crosstalk energy coupled between the wires in adjacent twisted pairs tends to be higher between twisted pairs that have the same twist lay and/or opposing directions as compared to other twisted pairs that have different twist lays and/or same twist directions. Thus, the unique twist lay serves to decrease the level of crosstalk between the adjacent twisted pairs within the cable. Therefore, twisted pairs within a cable are sometimes given unique twist lays when compared to other adjacent twisted pairs within the cable.
As the continuous twisted or helical structure reaches a termination point, for example as the cable is terminated to be joined to a connector, the helical structures of the individual twisted pairs are deformed to mate with contacts in the terminating hardware creating a detwisted region within the cable. The actual angle of arrival of the helix of the individual twisted pairs in relation to the mating hardware depends on where the cable is cut within its length. Therefore, the amount of deformation required to align the conductors of the wire pair with the connection points can vary from twisted pair to twisted pair within a cable. The random nature of the deformation of the helical structure creates undesirable inter-pair coupling variations from one connector to the next. Therefore, although the unique twist lay and twist direction can reduce the level of crosstalk within the cable, the de-twisting action produces a level of crosstalk that tends to be random.
In an attempt to reach cross-manufacturer compatibility, EIA/TIA mandates a known coupling level in category 5 mating hardware. Mating hardware is designed, via counter-coupling, to compensate for the mandated coupling level in order to establish a predetermined level of coupling in a data communications link over a category 5 cable. The variability in the inter-pair coupling encountered from one plug to the next serves to limit the effectiveness of the counter-coupling compensation.
This specified, standard level of coupling within the mating hardware is provided so that overall the system can have a level of crosstalk that ensures that the particular transmission standard is properly met. Although it is possible to reduce the actual amount of coupling in the mating hardware to improve overall performance, this is not desirable in order to be in compliance with the appropriate standards and reverse compatibility reasons as well. What is preferable is a constant, repeatable and known level of crosstalk. If a category 5 plug is connected to a superior performance jack, it is expected that the plug and jack will be able to meet category 5 coupling specifications. This means that the jack/plug must be able to counter-couple for the level of coupling specified for a category 5 plug/jack. In addition, if two superior performance connectors are used, it is reasonable to expect that the superior performance mating hardware is able to counter-couple for the level of coupling specified for the superior performance hardware.
It is desirable for the crosstalk occurring in the region adjacent to where the twisted pairs have exited from the cable be of a known, consistent, repeatable, and standard value in order to mate with the connecting hardware. At least part of the region is herein referred to as the xe2x80x9cdetwistedxe2x80x9d portion of the cable. Various conventional methods have been used in an attempt to improve the consistency of counter-coupling within the cable and jack or plug. For example, the use of shielded connectors, lead frames, and complex electronic counter-coupling have been used. However, these methods often increase the time required for installation, may require special tools, and can increase the material cost due to a larger parts count. This may lead to market acceptance problems due to the increased costs associated with the special tooling and the additional training required.
The EIA/TIA has mandated the values of the terminated open circuit crosstalk (TOC) which is assumed to represent the electrostatic component of the crosstalk. The TIA/EIA but has not yet mandated the values for terminated short circuit crosstalk(TSC) which is assumed to represent the magnetic component of crosstalk. Theory and models that support it predicts that the electrostatic coupling between wire pairs carries a phase offset of minus ninety (xe2x88x9290) degrees while magnetic coupling carries a phase offset of zero (0) or one hundred eighty (180) degrees. The models also depict that near end crosstalk and far end crosstalk are caused by the vectorial addition of these electrostatic and magnetic components, near end crosstalk having its current in reverse direction of far end crosstalk.
A fixture is provided adjacent to a cable containing multiple twisted pairs. The fixture includes longitudinal channels for controlling the trajectory of the wire pairs after the twisted pairs have been exited the cable and have been de-twisted. The fixture maintains the wire pairs in an orientation beneficial for reducing crosstalk.
In one aspect of the present invention a fixture for use with a multi-wire cable having a plurality of twisted pairs of insulated conductors, the cable having a proximal exit region adjacent to a de-twisted region wherein said twisted pair of insulated conductor exit said cable and are transitioned to a de-twisted configuration consisting of wire pairs, at least a portion of the fixture can be axially adjacent to the de-twisted region of the cable the wire pair in the detwisted region of the multi-wire cable are disposed within corresponding of the plurality of channels such that the trajectories of the plurality of wire pairs are controlled.
In one embodiment the channels are disposed on the exterior surface of the fixture and can be disposed in a helical pattern. In another embodiment the fixture is generally cylindrical and the longitudinal channels are disposed on diametrically opposed sides of the fixture. In another embodiment, the longitudinal channels are W-shaped.
In another aspect of the present invention the fixture is generally planar and has longitudinal channels disposed on a top surface and bottom surface of the fixture. In one embodiment, the longitudinal channels are arranged so as to orient the wire pairs in proper arrangement for mating with mating hardware. In one embodiment, there can be two longitudinal channels on the top surface that each have one wire of a wire pair disposed therein and the three longitudinal channels on the bottom each configured to contain both the wires of a wire pair. The longitudinal channels are arranged so as to properly arrange the wire pairs for mating with the mating hardware.